Subscriber data is stored in a central database within the network, the Home Location Register (HLR) or Home Subscriber Server (HSS). When a subscriber attaches to the network, the subscriber becomes registered in a SGSN-MME (Serving GPRS Support Node-Mobility Managing Entity) and/or MSC (Mobile Switching Center), which fetches a copy of the subscriber data and stores it locally in the local subscriber database in SGSN-MME, respectively in the Visitor Location Register (VLR) in the MSC. When a mobile entity, also called UE (user equipment) hereinafter, is under LTE radio coverage and has CSFB/SMS (Circuit Switched Fallback/Short Message Service) capability and sends an attach request to the MME, the MME will perform a combined attach and will register the UE for CS (circuit switched) services in one MSC-S(Server). The SGSN-MME and the MSC-S only serve subscribers that have a corresponding record in the own locally stored subscriber database. The serving SGSN-MME or MSC keeps track of the current location of the subscriber and stores the serving area (RAI/TAI (Routing Area Indicator, Tracking Area Identity in the SGSN-MME and LAI (Location Area Identity) in the MSC) in the locally stored subscriber database.
In case of CS voice terminating call, to reach a subscriber, a message is sent on a broadcasting channel within the last known location area (RAI/TAI or LAI) of the subscriber. The UE informs the controlling node when the location changes and additionally updates the information in regular intervals, for example in the range of a few hours (“periodic RAU/TAU or LAU” (Routing Area Update, Tracking Area Update, Location Area Update). If the area of the subscriber is not known, paging needs to be performed within the entire area (paging all RAs/TAs in SGSN-MME or “global paging” in MSC), which could be an entire “pool” area when several MMEs or MSCs form a pool. Network capacity for large-scale paging is limited, and the volume of such attempts is throttled, which may lead to unsuccessful call attempts.
If an UE tries to access network services in an SGSN-MME or an MSC that does not have a data record of the subscriber, service is denied. In that case, the UE has to re-attach to the network to re-gain service. If subscriber data is lost due to an outage, the subscriber is not reachable for terminating calls until before the UE subscriber data has been re-covered or the subscriber re-attaches to the network.
In one configuration each SGSN-MME or MSC serves a different part of a radio network and is connected to different radio network controllers. Furthermore, it is possible to arrange SGSN-MMEs or MSCs in a pooled configuration where they share a common set of radio network controllers serving a common section of radio network. This is indicated in FIG. 1 in more detail. In the upper part of FIG. 1 a plurality of non-pooled MSCs 10 are provided, each MSC being connected to one or more corresponding radio network controllers 11. Each MSC could control one or more radio network controllers. A radio network controller is connected to one MSC. Each radio network controller 11 controls a radio network controller area 12 with several location areas 13. In the lower part, a pooled configuration is shown in which a plurality of pooled MSCs 20 share a common set of radio network controllers 21. The set of radio network controllers 21 serves a common section 22 of the radio network comprising the different location areas 13.
A node capacity is typically dimensioned in a way that pooled nodes are in n+m redundancy. This means at an outage of up to m nodes, the remaining nodes in the pool can handle the entire traffic load.
One of the challenges in pooling SGSN-MMEs or MSCs is how to identify the serving node for originating and for terminating transactions.
A UE that is attached to a network that employs pooled network architecture is always attached to exactly one of the pooled controlling nodes in each access domain (one SGSN-MME in the PS domain and one MSC in the CS domain). Transactions involving the UE can be set up in either of two directions: from a remote network user through the network towards the served UE (henceforth referred to as “terminating transaction”) or from the served UE through the network towards a remote network user (referred to as “originating transaction”). A network user can be any telecommunication terminal that might get access to the telecommunication environment in order to initiate or receive calls, messages or any other telecommunication-related actions.
Typically, transactions are set up for voice calls or text messages (SMS).
For circuit switched terminating calls, a HLR keeps the controlling nodes addresses stored for every subscriber. In case of a terminating call, when the UE is located in the circuit switched domain, when the GMSC (Gateway MSC) requests routing information from the HLR, the HLR fetches a roaming number from the VLR of the MSC where the UE was registered during a combined or normal attach procedure.
Terminating transactions require the UE to be paged. To limit the coverage area within which paging needs to be performed, the last known area (RAI, TAI or LAI) is stored in the controlling node used for paging. This is symbolized by FIG. 2 where a HLR 15 keeps the VLR addresses which may be the corresponding controlling nodes where the VLRs are located, the controlling nodes being the pool of MSCs 20. Here, it is assumed that each MSC-S has exactly one co-located VLR. The HLR keeps the VLR address and fetches a roaming number (MSRN) from the VLR which is a routable number towards the serving MSC. For originating calls or location updates, the RNC 21 or the eNodeB sends messages that have an valid node indicator (GUMMEI (Globally Unique Mobility Management Entity Identifier) or NRI (Network Resource Identifier)) encoded in GUTI/P-TMSI/TMSI (Globally Unique Temporary Identifier, Preliminary Temporary Mobile Subscriber Identity) to one of the pooled SGSN-MME or MSC using weighted round robin. The weights of all SGSN-MMEs are administered in the RNC 21 or eNodeB and, for the MSC, are administered in the RNC 21. The RNC 21 or eNodeB sends messages that have a valid P-TMSI or GUTI to the SGSN-MME or respectively a valid TMSI to the MSC that is identified by the GUMMEI (for MME) or NRI (for SGSN and MSC).
Upon outage of a pooled SGSN-MME or MSC, another pooled SGSN-MME or MSC serves the subscriber after the next UE originating connection attempt, no automatic rebalancing is performed, once a failed control node is back to service.
For terminating calls, during the outage of a pooled SGSN-MME or MSC mobile terminating calls will fail, until the affected UE performs a location update or another originating connection attempt. Furthermore, for terminating calls, after the outage the UE remains unreachable until the next originating connection attempt from the UE.
For originating calls and location updates the situation is as follows:
The RNC or eNodeB uses supervision mechanisms to determine loss of activity towards an SGSN-MME or MSC which may take up to a few seconds. During the outage, after having determined loss of connectivity with the SGSN-MME or MSC that matches the GUMMEI or the NRI, upon the receiving of UE's first mobile originating transaction request, the radio controlling node, the RNC or eNodeB, sends messages that carry GUMMEI or NRI of the failed controlling node to one of the other similar controlling nodes (SGSN-MMEs for a failed SGSN-MME or MSC for a failed MSC) of the pool using a weighted round robin algorithm.
However, the transaction is rejected and the call attempt fails since the SGSN-MME/MSC on the receiving side does not recognize the GUMMEI or NRI. The UE then deletes the stored GUTI/P-TMSI/TMSI (with GUMMEI or NRI included) and performs a new attach/location update based on its IMSI (International Mobile Subscriber Identity). The RNC or eNodeB uses a weighted round robin to select an SGSN-MME or an MSC to send the message to, which registers the UE to that controlling node. If the UE is in the LTE domain and has CSFB/SMS capability, the MME will perform a combined attach and will register the UE also for circuit switched services in one MSC. The next originating or terminating call attempt will be then successful.
After outage without connection attempt from the UE, the first UE's originating connection attempt is (following the NRI) directed to the recovered SGSN-MME or MSC, which does no longer have the local subscriber database. When referring to the local subscriber database, both the local subscriber database in the SGSN-MME or the VLR database in the MSC are covered. The transaction is rejected and the call fails. The UE deletes the stored GUTI/P-TMSI/TMSI (with GUMMEI or NRI included) and performs a new attach/location update based on its IMSI. The RNC or eNodeB uses a weighted round robin algorithm to select an SGSN-MME or an MSC to send the message to, which registers the UE to that controlling node. If the UE is in the LTE domain and has CSFB/SMS capability, the MME will perform a combined attach and will register the UE for circuit switched services in one MSC. The next call attempt is successful.
This situation is summarized in FIG. 3 in which one of the MSC servers 20 experiences a disturbance so that the VLR data on this MSC is lost.
The above described situation has the following drawbacks. As discussed above, some terminating transactions terminating at a mobile entity will fail and further situations exist where originating transactions originating at the mobile entity will fail. Furthermore, to prevent identification of subscribers by eavesdropping on the radio interface, a mobile entity identifies itself by means of a temporary identification such as GUTI/P-TMSI or TMSI rather than a unique identification (IMSI) whenever possible. In the solutions discussed above, the UE's first originating connection attempt that was served by a pooled SGSN-MME or MSC before it fails will be rejected by the network and the mobile entity is forced to use the IMSI when reattaching to the network.
Furthermore, distribution of subscribers among the pool members is determined by the radio controllers which use weighted round robin with statically administered weight of pool members not taking actual load (for example number of served subscribers) of the pool members into consideration. This approach has drawbacks after recovery scenarios where the VLR data have been lost, when the recovered controller has low(er) load compared to the other controllers in the pool and so the load in the pool is not distributed evenly. Furthermore, the current mechanism leads to an imbalance of subscriber distribution between pool members. The imbalance continues to grow after recovery of the failed MSC, irrespective of the duration of the disturbance.